jQuery Validation plugin: how to check if an element is valid?

A little bit of context:

I'm using the jQuery Validation plugin to validate a sign-up form. I now want to implement an ajax call to check whether the user name is available in the system, and I want to make this ajax call only if the userName value is a valid one as per the rules set in $(form).validate();

I want something like:

$("#userName").keyup(function () {
    if ($("#userName").isValid()) {
        //make ajax called
    }
});

I searched the documentation but i couldn't identify the solution to my problem.